Modern snowmobiles typically have two skis for supporting the front of the machine and to provide steering and a rubber track for supporting the rear of the machine and to provide traction force. The rider is typically positioned anywhere from directly over the center of the track to over the front of the track, while the engine is located between the drive track and the skis, usually as low as possible.
A snowmobile is considered to be an off-road vehicle and as such it must be able to contend with many different riding environments. The terrain may change from smooth trails to large bumps and jumps. Snow conditions can vary from grippy hard-pack to bottomless powder, from smooth trails to rough and bumpy ones. The basic platform for a conventional snowmobile includes three components, a tunnel at the rear of the snowmobile, an engine cradle connected at the front of the tunnel, and a front suspension connected to the engine cradle. The three components are integrally connected to form a unitary structure that is rigid enough to withstand the forces impingent thereupon during operation of the snowmobile. A rear track suspension is mounted to the underside of the snowmobile within a tunnel that partially encloses the track and suspension. A seat for the snowmobile rider is provided on top of the tunnel and running boards are provided on either side for supporting the rider's feet.
One type of conventional tunnel is described in U.S. Patent Publication No. 2010/0288574, which shows the tunnel as essentially an inverted, U-shaped structure. The tunnel has a top portion, a left side portion, and a right side portion. A left side running board extends outwardly from the left side portion of the tunnel. Likewise, a right side running board (not shown) extends outwardly from the right side portion of the tunnel.
The conventional tunnel in the above-identified published patent application exemplifies other known tunnel configurations utilized in modern snowmobiles. For such tunnels, the left and right side portions are made from single panels or sheets. Because of the amount of load (e.g., static, dynamic, vibrational, high impact, etc.) carried by the chassis it is typically necessary to attach plates, doublers or other structural stiffening or reinforcing devices to the single panels for load path routing, local reinforcement reasons, or both. In some tunnel configurations, structural or weight removing features such as ribs, channels, or lightning holes may be formed in the single panels.